In construction projects utilizing concrete structural members such as cast-in-place reinforced concrete beams or columns in a multi-story structure, it is important to have the capability to measure in situ the compressive strength of particular structural members. In particular, it is often desirable to establish whether a lower limit strength has been achieved in critical members to assure that it is safe to remove temporary supplemental supports and/or impose additional loading during construction. Delaying unnecessarily the removal of support or the adding of such additional loading will lengthen the construction period and correspondingly increase project costs.
A common method for estimating in situ the strength of concrete structural members is the "pull out" test. This test normally calls for an anchor bolt with an appropriately sized transverse flange or extension to be embedded in the concrete before hardening. After the concrete has hardened, the exposed end of the anchor bolt is pulled and the force required to rupture the concrete is measured. A hydraulic jack, screw jack or the like is typically employed to apply the necessary pulling force to the anchor bolt, and necessarily exerts a reacting force against the surface of the concrete structure.
Several limitations and deficiencies in the pull out test have become apparent. First, in order to limit the amount of required pull-out force and to preclude a non-indicative local crushing failure, it is necessary to embed the anchor bolt near the surface of the concrete and the results of the test therefore may not be representative of the overall concrete structure. Second, the pull-out forces tend to impose shear and tensile stresses along the potential rupture surface, whereas the actual stresses imposed on a concrete member under load tend to be shear and compressive. Third, the pull-out forces are generally imposed in a direction transverse to the direction of actual critical forces imposed under load. Fourth, a relatively large force is required to pull out the anchor bolt so that a specially fabricated jacking and force measuring device is needed. Such devices tend to be expensive and prone to maladjustment or damage under adverse field conditions. Fifth, the jacking device requires a smooth, flat and hard bearing surface to react against, thus requiring special precautions during casting and construction. Sixth, the anchor bolt must extend perpendicular to the bearing surface, thus requiring precautions to assure that the bolt is not tipped or dislodged during the pouring and setting of the concrete. Finally, the pull out test may cause a conical piece of material to be broken away at the surface of the structural member which is unsightly and may require repair or filling.
It is the object of the present invention to provide a device and procedure for measuring in situ the compressive strength of concrete structural elements which overcomes and circumvents the various deficiencies and limitations in the prior art.